The nonspecific binding of Fe3+ to transferrin in the absence of synergistic anions

Exploring titanium(IV) chemical proximity to iron(III) to elucidate a function for Ti(IV) in the human body

Despite its natural abundance and widespread use as food, paint additive, and in bone implants, no specific biological function of titanium is known in the human body. High concentrations of Ti(IV) could result in cellular toxicity, however, the absence of Ti toxicity in the blood of patients with titanium bone implants indicates the presence of one or more biological mechanisms to mitigate toxicity. Similar to Fe(III), Ti(IV) in blood binds to the iron transport protein serum transferrin (sTf), which gives credence to the possibility of its cellular uptake mechanism by transferrin-directed endocytosis. However, once inside the cell, how sTf bound Ti(IV) is released into the cytoplasm, utilized, or stored remain largely unknown. To explain the molecular mechanisms involved in Ti use in cells we have drawn parallels with those for Fe(III). Based on its chemical similarities with Fe(III), we compare the biological coordination chemistry of Fe(III) and Ti(IV) and hypothesize that Ti(IV) can bind to similar intracellular biomolecules. The comparable ligand affinity profiles suggest that at high Ti(IV) concentrations, Ti(IV) could compete with Fe(III) to bind to biomolecules and would inhibit Fe bioavailability. At the typical Ti concentrations in the body, Ti might exist as a labile pool of Ti(IV) in cells, similar to Fe. Ti could exhibit different types of properties that would determine its cellular functions. We predict some of these functions to mimic those of Fe in the cell and others to be specific to Ti. Bone and cellular speciation and localization studies hint toward various intracellular targets of Ti like phosphoproteins, DNA, ribonucleotide reductase, and ferritin. However, to decipher the exact mechanisms of how Ti might mediate these roles, development of innovative and more sensitive methods are required to track this difficult to trace metal in vivo.

Unusual Synergism of Transferrin and Citrate in the Regulation of Ti(IV) Speciation, Transport, and Toxicity

Human serum transferrin (sTf) is a protein that mediates the transport of iron from blood to cells. Assisted by the synergistic anion carbonate, sTf transports Fe(III) by binding the metal ion in a closed conformation. Previous studies suggest sTf's role as a potential transporter of other metals such as titanium. Ti is a widely used metal in colorants, foods, and implants. A substantial amount of Ti is leached into blood from these implants. However, the fate of the leached Ti and its transport into the cells is not known. Understanding Ti interaction with sTf assumes a greater significance with our ever increasing exposure to Ti in the form of implants. On the basis of in vitro studies, it was speculated that transferrin can bind Ti(IV) assisted by a synergistic anion. However, the role and identity of the synergistic anion(s) and the conformational state in which sTf binds Ti(IV) are not known. Here we have solved the first X-ray crystal structure of a Ti(IV)-bound sTf. We find that sTf binds Ti(IV) in an open conformation with both carbonate and citrate as synergistic anions at the metal binding sites, an unprecedented role for citrate. Studies with cell lines suggest that Ti(IV)-sTf is transported into cells and that sTf and citrate regulate the metal's blood speciation and attenuate its cytotoxic property. Our results provide the first glimpse into the citrate-transferrin synergism in the regulation of Ti(IV) bioactivity and offers insight into the future design of Ti(IV)-based anticancer drugs.

Phosphate inhibits in vitro Fe3+ loading into transferrin by forming a soluble Fe(III)-phosphate complex: a potential non-transferrin bound iron species

In chronic kidney diseases, NTBI can occur even when total iron levels in serum are low and transferrin is not saturated. We postulated that elevated serum phosphate concentrations, present in CKD patients, might disrupt Fe(3+) loading into apo-transferrin by forming Fe(III)-phosphate species. We report that phosphate competes with apo-transferrin for Fe(3+) by forming a soluble Fe(III)-phosphate complex. Once formed, the Fe(III)-phosphate complex is not a substrate for donating Fe(3+) to apo-transferrin. Phosphate (1-10mM) does not chelate Fe(III) from diferric transferrin under the conditions examined. Complexed forms of Fe(3+), such as iron nitrilotriacetic acid (Fe(3+)-NTA), and Fe(III)-citrate are not susceptible to this phosphate complexation reaction and efficiently deliver Fe(3+) to apo-transferrin in the presence of phosphate. This reaction suggests that citrate might play an important role in protecting against Fe(III), phosphate interactions in vivo. In contrast to the reactions of Fe(3+) and phosphate, the addition of Fe(2+) to a solution of apo-transferrin and phosphate lead to rapid oxidation and deposition of Fe(3+) into apo-transferrin. These in vitro data suggest that, in principle, elevated phosphate concentrations can influence the ability of apo-transferrin to bind iron, depending on the oxidation state of the iron.

The long history of iron in the Universe and in health and disease

BACKGROUND

Not long after the Big Bang, iron began to play a central role in the Universe and soon became mired in the tangle of biochemistry that is the prima essentia of life. Since life's addiction to iron transcends the oxygenation of the Earth's atmosphere, living things must be protected from the potentially dangerous mix of iron and oxygen. The human being possesses grams of this potentially toxic transition metal, which is shuttling through his oxygen-rich humor. Since long before the birth of modern medicine, the blood-vibrant red from a massive abundance of hemoglobin iron-has been a focus for health experts.

SCOPE OF REVIEW

We describe the current understanding of iron metabolism, highlight the many important discoveries that accreted this knowledge, and describe the perils of dysfunctional iron handling.

GENERAL SIGNIFICANCE

Isaac Newton famously penned, "If I have seen further than others, it is by standing upon the shoulders of giants". We hope that this review will inspire future scientists to develop intellectual pursuits by understanding the research and ideas from many remarkable thinkers of the past.

MAJOR CONCLUSIONS

The history of iron research is a long, rich story with early beginnings, and is far from being finished. This article is part of a Special Issue entitled Transferrins: Molecular mechanisms of iron transport and disorders.

The ferroxidase center is essential for ferritin iron loading in the presence of phosphate and minimizes side reactions that form Fe(III)-phosphate colloids

Ferritin iron loading was studied in the presence of physiological serum phosphate concentrations (1 mM), elevated serum concentrations (2-5 mM), and intracellular phosphate concentrations (10 mM). Experiments compared iron loading into homopolymers of H and L ferritin with horse spleen ferritin. Prior to studying the reactions with ferritin, a series of control reactions were performed to study the solution chemistry of Fe(2+) and phosphate. In the absence of ferritin, phosphate catalyzed Fe(2+) oxidation and formed soluble polymeric Fe(III)-phosphate complexes. The Fe(III)-phosphate complexes were characterized by electron microscopy and atomic force microscopy, which revealed spherical nanoparticles with diameters of 10-20 nm. The soluble Fe(III)-phosphate complexes also formed as competing reactions during iron loading into ferritin. Elemental analysis on ferritin samples separated from the Fe(III)-phosphate complexes showed that as the phosphate concentration increased, the iron loading into horse ferritin decreased. The composition of the mineral that does form inside horse ferritin has a higher iron/phosphate ratio (~1:1) than ferritin purified from tissue (~10:1). Phosphate significantly inhibited iron loading into L ferritin, due to the lack of the ferroxidase center in this homopolymer. Spectrophotometric assays of iron loading into H ferritin showed identical iron loading curves in the presence of phosphate, indicating that the ferroxidase center of H ferritin efficiently competes with phosphate for the binding and oxidation of Fe(2+). Additional studies demonstrated that H ferritin ferroxidase activity could be used to oxidize Fe(2+) and facilitate the transfer of the Fe(3+) into apo transferrin in the presence of phosphate.

Enhancement of iron-catalyzed lipid peroxidation by acidosis in brain homogenate: comparative effect of diphenyl diselenide and ebselen

Iron is more soluble at lower pH values; therefore we hypothesized that decreasing the environmental pH would lead to increased iron-mediated lipid peroxidation. Diphenyl diselenide and ebselen are potential candidates as neuroprotective agent, particularly in situations involving overproduction of free radicals and involving cellular pH fall. The aim of the present study was (a) to investigate the relationship between lipid peroxidation and acidosis in brain homogenate and (b) to test the influence of pH on the antioxidant properties of diphenyl diselenide and ebselen. For the purpose rat brain homogenate was incubated at different pH ranging from physiological to acidic values and extent of lipid peroxidation was measured. Thiobarbituric acid-reactive species (TBARS) production significantly increased when homogenate was incubated in the pH (5.4-6.8) medium both in the absence and presence of Fe (II) as compared with physiological pH (7.4). These data indicate that lipid peroxidation processes, mediated by iron, are enhanced with decreasing extracellular pH. The iron mobilized may come from reserves where it is weakly bound. Diphenyl diselenide significantly protected TBARS production at all studied pH values while ebselen offered only a small statistically non-significant protection. However, calculated IC(50) for TBARS inhibition indicated that pH did not change anti-oxidant activities of the tested compounds. This study provides in-vitro evidence for acidosis induced oxidative stress in brain homogenate and anti-oxidant action of diphenyl diselenide.

Differential response of iron metabolism to oxidative stress generated by antimycin A and nitrofurantoin

The close interrelationship of oxidative stress and iron is evident by the influence of intracellular reactive oxygen species on iron metabolism. Oxygen radicals can lead to release of iron from iron-sulfur proteins and ferritin, and can damage iron-containing enzymes such as mitochondrial aconitase. Treatment of HepG2 human hepatoma cells with antimycin A has two effects relating to iron depending on the concentrations of antimycin A: increase of the labile iron pool and stimulation of non-transferrin-bound iron uptake. Whereas the first could also be generated with nitrofurantoin, the stimulation of non-transferrin-bound iron uptake was only seen with antimycin A and needed considerably higher concentrations. Pretreatment of the cells with ebselen, which scavenges peroxides, reverted only the effect of nitrofurantoin on the labile iron pool. Depletion with iron chelators before or after treatment with antimycin A diminished the stimulation of non-transferrin-bound iron uptake. We conclude that the generation of oxygen radicals in the mitochondria leads to the liberation of iron from mitochondrial enzymes, which enters the labile iron pool. But high concentrations of antimycin A leading to the stimulation of non-transferrin-bound iron uptake is possibly not related to the inhibition of the respiratory chain.

Friedreich's ataxia, no changes in mitochondrial labile iron in human lymphoblasts and fibroblasts: a decrease in antioxidative capacity?

Friedreich's ataxia (FRDA) is caused by low expression of frataxin, a small mitochondrial protein. Studies with both yeast and mammals have suggested that decreased frataxin levels lead to elevated intramitochondrial concentrations of labile (chelatable) iron, and consequently to oxidative mitochondrial damage. Here, we used the mitochondrion-selective fluorescent iron indicator/chelator rhodamine B-[(1,10-phenanthrolin-5-yl)aminocarbonyl]benzylester (RPA) to determine the mitochondrial chelatable iron of FRDA patient lymphoblast and fibroblast cell lines, in comparison with age- and sex-matched control cells. No alteration in the concentration of mitochondrial chelatable iron could be observed in patient cells, despite strongly decreased frataxin levels. Uptake studies with (55)Fe-transferrin and iron loading with ferric ammonium citrate revealed no significant differences in transferrin receptor density and iron responsive protein/iron regulatory element binding activity between patients and controls. However, sensitivity to H(2)O(2) was significantly increased in patient cells, and H(2)O(2) toxicity could be completely inhibited by the ubiquitously distributing iron chelator 2,2'-dipyridyl, but not by the mitochondrion-selective chelator RPA. Our data strongly suggest that frataxin deficiency does not affect the mitochondrial labile iron pool or other parameters of cellular iron metabolism and suggest a decreased antioxidative defense against extramitochondrial iron-derived radicals in patient cells. These results challenge current concepts favoring the use of mitochondrion-specific iron chelators and antioxidants to treat FRDA.

The Oxalate Effect on Release of Iron from Human Serum Transferrin Explained

A unique feature of the mechanism of iron binding to the transferrin (TF) family is the synergistic relationship between metal binding and anion binding. Little or no iron will bind to the protein without concomitant binding of an anion, physiologically identified as carbonate. Substitution of oxalate for carbonate produces no significant changes in polypeptide folding or domain orientation in the N-lobe of human serum TF (hTF) as revealed by our 1.2A structure. The oxalate is able to bind to the iron in a symmetric bidentate fashion, which, combined with the low pK(a) of the oxalate anion, makes iron displacement more difficult as documented by both iron release kinetic and equilibrium data. Characterization of an N-lobe in which the arginine at position 124 is mutated to alanine reveals that the stabilizing effect of oxalate is even greater in this mutant and nearly cancels the destabilizing effect of the mutation. Importantly, incorporation of oxalate as the synergistic anion appears to completely inhibit removal of iron from recombinant full-length hTF by HeLa S(3) cells, strongly indicating that oxalate also replaces carbonate in the C-lobe to form a stable complex. Kinetic studies confirm this claim. The combination of structural and functional data provides a coherent delineation of the effect of oxalate binding on hTF and rationalizes the results of many previous studies. In the context of iron uptake by cells, substitution of carbonate by oxalate effectively locks the iron into each lobe of hTF, thereby interfering with normal iron metabolism.

Mechanisms of TfR-mediated transcytosis and sorting in epithelial cells and applications toward drug delivery

Transferrin receptor has been an important protein for many of the advances made in understanding the intricacies of the intramolecular sorting pathways of endocytosed molecules. The unique internalization and recycling functions of transferrin receptor have also made it an attractive choice for drug targeting and delivery of large protein-based therapeutics and toxins. Recent advances in elucidating the role of the intracellular controllers of transferrin recycling and sorting, such as Rab proteins and their effectors, have led to enhancement of transferrin receptor as a drug delivery vehicle. This review focuses on the use of transferrin receptor as an agent for facilitating drug delivery and targeting, and the role that mechanisms of transferrin receptor sorting and transcytosis play in these events.

Synergistic anion and metal binding to the ferric ion-binding protein from Neisseria gonorrhoeae

The 34-kDa periplasmic iron-transport protein (FBP) from Neisseria gonorrhoeae (nFBP) contains Fe(III) and (hydrogen)phosphate (synergistic anion). It has a characteristic ligand-to-metal charge-transfer absorption band at 481 nm. Phosphate can be displaced by (bi)carbonate to give Fe.CO(3).nFBP (lambda(max) 459 nm). The local structures of native Fe-PO(4)-nFBP and Fe.CO(3).nFBP were determined by EXAFS at the FeK edge using full multiple scattering analysis. The EXAFS analysis reveals that both phosphate and carbonate ligands bind to FBP in monodentate mode in contrast to transferrins, which bind carbonate in bidentate mode. The EXAFS analysis also suggests an alternative to the crystallographically determined position of the Glu ligand, and this in turn suggests that an H-bonding network may help to stabilize monodentate binding of the synergistic anion. The anions oxalate, pyrophosphate, and nitrilotriacetate also appear to serve as synergistic anions but not sulfate or perchlorate. The oxidation of Fe(II) in the presence of nFBP led to a weak Fe(III).nFBP complex (lambda(max) 471 nm). Iron and phosphate can be removed from FBP at low pH (pH 4.5) in the presence of a large excess of citrate. Apo-FBP is less soluble and less stable than Fe.nFBP and binds relatively weakly to Ga(III) and Bi(III) but not to Co(III) ions, all of which bind strongly to apo-human serum transferrin.

Al and Si: their speciation, distribution, and toxicity

OBJECTIVES

In dialysis patients both aluminum (AI) and silicon (Si) may accumulate. Whereas the toxic effects of AI within this population are clearly established, little is known on the role of Si in the development/protection of particular dialysis-related diseases. A clear insight in the protein binding and speciation of trace elements is important to better understand the mechanisms underlying their toxicity/essentiality. Research in this field however is complex and often prone to analytical difficulties and inaccuracies.

DESIGN AND METHODS

In the first part of this review techniques used for speciation studies of AI and Si in biological fluids are discussed. Notwithstanding recent technical advances (a) extraneous metal contamination, (b) unrecognized aspecific binding of metals to proteins, and (c) unwanted interactions with separation equipment such as chromatography columns and ultrafiltration membranes remain important pitfalls and often lead to erroneous conclusions. The factors that determine the speciation of AI and Si and their ultimate tissue distribution and toxicity are dealt with in the second part. Here, experimental data obtained with various speciation techniques are linked to in vivo data on the tissue distribution, localization/toxicity of both elements.

CONCLUSIONS

A model in which the AI tissue distribution/toxicity is mediated by either its citrate or transferrin bound form is proposed.

Low molecular weight iron in cerebral ischemic acidosis in vivo

BACKGROUND AND PURPOSE

Iron-catalyzed radical generation is a potentially significant mechanism by which extensive tissue acidosis exacerbates brain injury during ischemia/reperfusion. We hypothesized that levels of low-molecular-weight (LMW) iron increase during in vivo global cerebral ischemia in a pH-dependent manner, potentially catalyzing oxidant injury. The present study quantified regional differences in LMW iron during global cerebral incomplete ischemia and determined whether augmenting the fall in ischemic tissue pH with hyperglycemia also amplifies free iron availability.

METHODS

Dogs anesthetized with pentobarbital-fentanyl were treated with 30 minutes of global incomplete cerebral ischemia produced by intracranial pressure elevation. Cerebral energy metabolites (ATP, phosphocreatine) and intracellular pH (pHi) were measured by 31P magnetic resonance spectroscopy. Preischemic plasma glucose level was manipulated to titrate end-ischemic pHi. After ischemia, brains were perfused with cold phosphate-buffered saline solution; then 16 different brain areas were sampled, filtered to separate the LMW fraction (<30000 D), and assayed by rapid colorimetric assay for tissue iron. Total iron, LMW iron, and protein in each sample were measured in sham-operated (no ischemia, n=8), normoglycemic ischemia (ISCH [glucose 7+/-4 mmol/L], n=7), and hyperglycemic (GLU-ISCH [glucose 31+/-3 mmol/L], n=9) groups.

RESULTS

High-energy phosphates fell to near zero values in both ISCH and GLU-ISCH groups by 30 minutes but remained unchanged in the sham-operated group. As expected, pHi decreased during ischemia but to a greater extent in GLU-ISCH (6.20+/-0.05 in ISCH, 6.08+/-0.04 in GLU-ISCH, P<.05). Iron could be detected in all areas of the brain in sham-operated animals, with the highest amounts obtained from subcortical areas such as the hippocampus, pons, midbrain, and medulla. Total iron was higher in ISCH relative to sham-operated animals and higher in cortex and pons relative to GLU-ISCH. Regional LMW (as a percentage of total iron; LMW/total iron) was elevated in numerous brain areas in ISCH, including cortical gray matter, cerebellum, hippocampus, caudate, and midbrain. LMW/total iron was higher in GLU-ISCH versus ISCH in cortical gray matter only. In other brain areas, ischemic LMW/total iron was equivalent in glucose-treated or normoglycemic animals (white matter, thalamus, pons, medulla) or lower in the glucose-treated group (cerebellum, hippocampus, caudate, midbrain).

CONCLUSIONS

These data demonstrate that levels of total and LMW iron increase with global cerebral ischemia in the majority of cortical and subcortical regions of normoglycemic brain. However, exacerbation of ischemic acidosis via glucose administration does not increase tissue iron and produces a greater increase in the LMW fraction in cortical gray matter only. In other brain regions, total and LMW iron availability is similar to that of nonischemic animals.

Deferoxamine reduces early metabolic failure associated with severe cerebral ischemic acidosis in dogs

BACKGROUND AND PURPOSE

Postischemic metabolic injury may be mediated by acidosis and tissue bicarbonate depletion, with consequent-iron mobilization and oxygen radical formation during reperfusion. We have previously shown that reducing intracellular pH to below 5.7 and bicarbonate ion to below 1 to 2 mmol/L during hyperglycemic ischemia produces a profound secondary deterioration of brain ATP and cerebral blood flow during reperfusion. This study tested the hypothesis that pretreatment with free deferoxamine ameliorates metabolic decay and delayed hypoperfusion after global hyperglycemic ischemia. In addition, deferoxamine conjugated to a high-molecular-weight starch was administered to determine the importance of an intravascular site of action. Iron-loaded deferoxamine was used to determine whether the iron chelation properties of deferoxamine are important to postischemic viability as distinguished from the agent's significant radical scavenging potential.

METHODS

Cerebral ATP, phosphocreatine, and pH were measured by 31P magnetic resonance spectroscopy in anesthetized dogs. Tissue bicarbonate concentration was calculated from the Henderson-Hasselbalch equation. Incomplete cerebral ischemia was produced by intracranial pressure elevation for 30 minutes with plasma glucose at 540 +/- 15 mg/dL. Free deferoxamine, saline vehicle, hydroxyethyl starch-conjugated deferoxamine, hydroxyethyl starch vehicle, and deferoxamine loaded with equimolar ferric chloride were administered intravenously in five groups of dogs. The dose of deferoxamine was 50 mg/kg before ischemia, 50 mg/kg at the onset of reperfusion, and 50 mg/kg over the 180-minute reperfusion period.

RESULTS

Ischemic hemispheric blood flow (mean, 6 to 8 mL/min per 100 g), intracellular pH (5.7 to 6.0), and bicarbonate levels (1 to 2 mmol/L) were similar in all groups. During reperfusion, cerebral pH and bicarbonate recovered only in the free-deferoxamine group. Both ATP and phosphocreatine initially increased in all groups, but recovery was sustained only in the free-deferoxamine group. Secondary losses of energy phosphates and cerebral oxygen consumption were observed in all other groups, accompanied by progressive reduction of perfusion.

CONCLUSIONS

These data support the hypothesis that iron catalyzed oxygen radical production plays an important role in acidosis-mediated mechanisms of ischemic brain injury. The results with free and iron-loaded deferoxamine suggest that iron scavenging is an important, but not necessarily the principal, component of this mechanism. The poor recovery seen with conjugated deferoxamine indicates that the beneficial action of deferoxamine is not localized within the intravascular compartment.

A new approach to quantitate carbohydrate-deficient transferrin isoforms in alcohol abusers: partial iron saturation in isoelectric focusing/immunoblotting and laser densitometry

Carbohydrate-deficient transferrin (Tf) represents a significant advance over previous markers of alcohol abuse. Isoelectric focusing (IEF) analysis of affinity-purified Tf, under conditions of total iron saturation, identifies a major isoform at pI 5.4 in both normal consumers and alcohol abusers; three additional Tf isoforms (pI 5.6, 5.7, and 5.8) are associated with alcohol abuse. Under conditions of partial iron saturation, IEF analysis of affinity-purified Tf reveals up to seven isoforms (pI range 5.3-6.0) common to normal consumers and alcohol abusers; three additional transferrin isoforms (pI range 6.1-6.3) are present in 68% (15/22) of the alcohol abuser specimens, but in only 8% (1/12) of the specimens from normal consumers and in none of the three specimens from abstainers. These three diagnostic bands comigrate with a set of defined Tf isoforms: human iron-free Tf containing two sialic acid residues, human sialic acid-free Tf with one iron molecule, and human sialic acid-free, iron-free Tf. Serum specimens from normal consumers and alcohol abusers, analyzed for Tf isoforms by an IEF-immunoblot method under conditions of partial iron saturation, expressed Tf isoforms similar to those found using affinity-purified Tf in standard IEF. Visual examination of the immunoblots reveals the diagnostic bands in 67% (32/48) of patients with histories of sustained alcohol abuse compared with only 17% (8/48) of the normal consumers. Scanning densitometry and volume integration analysis of the immunoblots representative of normal consumer and alcohol abuser populations results in mean (+/- SE) values of 4.1 +/- 0.8 and 19.3 +/- 3.6 units, respectively (p < 0.0002).

A transferrin-independent iron uptake activity in Plasmodium falciparum-infected and uninfected erythrocytes

Non-heme iron is essential for the asexual growth of the human malaria parasite Plasmodium falciparum in mature erythrocytes. Utilization of iron bound to serum transferrin by the parasitized cells has been postulated, but direct evidence for its specific delivery has not been reported. Here we demonstrate that normal levels of transferrin in human serum are not required for intraerythrocytic P. falciparum growth: culture medium immunodepleted 500-1000 fold in human transferrin was capable of supporting parasitemias and rates of invasion comparable to those observed in non-depleted medium. 55Fe bound to transferrin was not taken up by infected cells. A transferrin-independent non-heme iron uptake activity was, however, detected in both infected and uninfected erythrocytes when iron was presented to the cells as 55Fe-NTA or 55Fe-citrate. Although the uptake activity was not parasite specific, the radiolabel was found in association with parasites mechanically released from the infected erythrocytes, indicating that it is delivered to the intracellular organism. Evidence is presented that the transferrin-independent iron uptake activity is time-, temperature- and concentration-dependent, but apparently not energy-dependent.

Fe3+ binding to ovotransferrin in the presence of α-amino acids

The ability of L-alpha-amino acids as synergistic anions for iron binding to ovotransferrin was investigated through electronic spectroscopy. Glycine and glutamic acid were found to form by far the most stable ternary Fe(3+)-ovotransferrin-amino acid complexes. Less stable adducts were formed by amino acids with a hydroxy, amide or sulphur-containing group in the side chain, while the complexes with leucine, isoleucine, valine, lysine, arginine, tyrosine and tryptophan failed to form. Evidence is obtained that the synergistic effectiveness of the H2N-CH-COO- moiety is determined not only by the isoelectric point of the amino acid and the steric hindrance of its side chain, but a significant role is also played by interactions of the side chain itself with residues in the metal binding domains. Zn2+, Cd2+ and Co2+ are found to bind to ovotransferrin in the presence of glycine. 113Cd-NMR spectra on the Cd-derivative indicate that, according to the interlocking-sites model, the amino group of glycine directly binds to the metal ion.

Oxalate as synergistic anion for Cd(II) binding to ovotransferrin

The Cd(II) derivative of ovotransferrin containing oxalate as synergistic anion was investigated through 113Cd- and 13C-NMR spectroscopy and compared with the analogous derivative obtained in the presence of bicarbonate. Cadmium loaded in the C site gives rise to a 113Cd-NMR signal at 54 ppm, while that bound to the N site is broadened beyond detection. The two resonances at 168.5 and 169.9 ppm observed in the 13C-NMR spectrum of the Cd2 derivative obtained with [13C]oxalate each correspond to slowly exchanging oxalate specifically bound to a single site. No splitting of these resonances due to 113Cd-13C magnetic coupling is observed upon insertion of 113Cd-enriched Cd(II) ion, unlike previous observations for the corresponding derivative obtained with bicarbonate as synergistic anion. It was found that the metal sites in the present derivative are inequivalent, as observed for other metal-transferrin-oxalate adducts. The C site is found to be sensitive to a residue ionization with a pKa of 9.5.

The displacement of copper by iron at the specific binding sites of ovotransferrin

We have examined the kinetics and mechanism by which iron can displace copper at the specific metal-binding sites of ovotransferrin. Fe2+ was added to Cu2+-ovotransferrin-CO3(2-) in the presence of NaHCO3 and ambient O2. The reaction has been followed by standard and stopped-flow spectrophotometry, EPR spectroscopy and analysis of chromogen-reactive Fe2+. The reaction is best described as triphasic. An initial jump in absorbance takes place in the first 2 s. In the next minute there is a further increase in absorbance and shift in the spectral maximum from 440 to 446 nm. The third phase is complex. The bulk of the spectrophotometric change, a decrease in absorbance with a shift to a maximum of 453 nm, lasts approx. 3 min. Minor spectral and EPR changes, however, take place over the next several hours. Chromogenic analysis of Fe2+ indicates that approx. 1 min is required to oxidize the Fe2+. EPR spectra reveal the formation of an Fe3+-ovotransferrin complex within the first 20 s; however, this lacks the characteristic doublet of specific Fe3+-ovotransferrin-CO3(2-). The simultaneous presence of specific Cu2+-ovotransferrin-CO3(2-) and Fe3+-ovotransferrin-CO3(2-) signals suggests a period in which the protein specifically binds both metal ions perhaps resulting from a differential reactivity of the two metal-binding sites. The addition of Cu(NO3)2 to Fe3+-ovotransferrin-CO3(2-) resulted in a complex with specific Fe3+ and non-specific Cu2+. The EPR spectrum of this complex and the final product of our displacement reaction were virtually identical. Distinct parallels in reaction of Cu2+-ovotransferrin-CO3(2-) with Fe(NH4)2(SO4)2, Fe(NO3)3 and Fe3+-nitrilotriacetic acid were observed. A reaction sequence involving the binding and oxidation of non-specific Fe2+ followed by Cu2+ displacement by Fe3+ at the specific sites and binding of non-specific Cu2+ is suggested.

Magnesium potentiation of iron-transferrin binding

The binding of iron to transferrin was studied by loading iron (III) onto apotransferrin in a chloride and a nitrilotriacetate form. When magnesium was added, a marked increase occurred in both the rate of iron binding and the maximum level of iron loaded on transferrin utilizing either iron salt. In the absence of magnesium the amount of iron required to achieve 50 percent saturation of the binding sites was 1.6 x 10(-4) M, whereas when magnesium was added, only about one-third as much iron (0.54 x 10(-4) M) was required. These data suggest an allosteric effect on transferrin by magnesium which potentiates iron (III) binding.

The spectroelectrochemical determination of the reduction potential of diferric serum transferrin

The first spectroelectrochemical measurement of the formal reduction potential of iron transferrin has been carried out using methyl viologen to mediate electron transfer to the protein. These calculations take into consideration the weak nature of the ferrous transferrin complex. A value of -0.52(8) V vs. the normal hydrogen electrode was obtained in 0.100 M tris(hydroxymethyl)aminomethane buffer at pH 7.4, 22 degrees C, and 2.0 M KCl. A high ionic strength was necessary to effect reduction, supporting the observation that ions play an important role in the reduction of iron in transferrin. Finally, a procedure for carrying out the reduction of methyl viologen at a gold electrode in a spectrophotometric cell is described.

The influence of inorganic anions on the formation and stability of Fe3+-transferrin-anion complexes

Harris (Biochemistry 24 (1985) 7412) reports that inorganic anions bind to human apotransferrin in such a way as to perturb the ultraviolet spectrum. The locus of binding is thought to involve the specific metal/anion-binding sites since no perturbation is observed with Fe3+-transferrin-CO3(2-). Paradoxically, we were unable to demonstrate the formation of Fe3+-transferrin-inorganic anion complexes despite the presence of high concentrations of SO4(2-), H2PO4-, Cl-, ClO4- or NO3-. Similar results were found for human lactoferrin. Electron paramagnetic resonance spectroscopy and visible spectrophotometry were used to monitor the results. An attempt to form the H2PO4- complex by displacement of glycine from Fe3+-transferrin-glycine resulted only in the disruption of the ternary complex. A series of inorganic anions varied in their ability to release iron from Fe3+-transferrin-CO3(2-) at pH 5.5, the approximate pH of endosomes where iron release takes place within cells. The order of effectiveness was H2P2O7(2-) much greater than H2PO4- greater than SO4(2-) greater than NO3- greater than Cl- greater than ClO4-. The rate of iron removal from Fe3+-transferrin-CO3(2-) at pH 5.5 by a 4-fold excess of pyrophosphate was greatly enhanced by physiological NaCl concentration. Iron removal was complete within 10 min, the approximate time for iron release from Fe3+-transferrin-CO3(2-) in developing erythroid cells. Thus, inorganic anions may have a significant effect on the release of iron under physiological conditions despite the fact that such inorganic anions cannot act as synergistic anions. The results are discussed in relation to a special role for the carboxylate group in allowing ternary complex formation.

Iron transfer from ferritin to transferrin. Effect of serum factors

The transfer of iron from 59Fe-labelled human spleen ferritin to human apotransferrin occurs in the absence of either reducing or chelating agents. The reaction is first order with respect to ferritin and zero order to apotransferrin. The transfer is enhanced by low-Mr substances from human serum such as ascorbate, citrate, bicarbonate and lactate. A mixture of the four molecules at their normal physiological concentrations can increase the iron exchange to the same extent as that observed with an ultrafiltrate of serum. A pathway of intracellular iron mobilization is considered.

A protein on Plasmodium falciparum-infected erythrocytes functions as a transferrin receptor

Several observations suggest that iron is essential for the development of malaria parasites but there is evidence that the parasites in erythrocytes do not obtain iron from haemoglobin. The total haemin level in parasitized erythrocytes does not vary during parasite development, indicating that the iron-containing moiety of haemoglobin is not detectably metabolized. Although parasite proteases can degrade the protein part of haemoglobin in red cells, no parasite enzymes that degrade haemin have been identified. In mammalian cells, haemin is degraded to carbon monoxide and bilirubin by the enzyme haeme oxygenase. This enzyme has not been found in malaria parasites. In fact haemin has been found to be toxic to parasite carbohydrate metabolism. Thus, iron apparently cannot be liberated from haemin and instead is sequestered in infected red cells as haemozoin, the characteristic pigment associated with malarial infection. If iron bound to transferrin is the source of ferric ions for malaria parasites within mature erythrocytes, then the parasite must synthesize its own transferrin receptor and localize it on the surface of the infected cell, because the receptors for transferrin are lost during erythrocyte maturation. Our results here suggest that Plasmodium falciparum synthesizes its own transferrin receptors enabling it to take up iron from transferrin by receptor-mediated endocytosis.

The study of iron mobilisation from transferrin using alpha-ketohydroxy heteroaromatic chelators

A group of heteroaromatic chelators with an alpha-ketohydroxy binding site have been tested for their ability to mobilise iron from transferrin in vitro. When these chelators were mixed with iron-saturated transferrin at physiological pH, biphasic reactions were observed. The alpha-ketohydroxy heteroaromatic chelators were found to cause substantial iron removal compared to other known chelators. These findings suggest that these chelators may have an important role in the study of iron metabolism and a possible clinical use in the treatment of transfusional iron overload in thalassaemia, and other diseases of iron imbalance.

Role of metals in oxygen radical reactions

Partially-reduced forms of dioxygen or "oxy-radicals" (superoxide, O2-/HO2; hydrogen peroxide, H2O2; hydroxyl radical X OH) and oxidants of comparable reactivity are implicated in an increasing number of physiological, toxicological, and pathological states. Transition metal catalysis is recognized as being integral to the generation and the reactions of these activated oxygen species. Factors such as pH and chelation govern the reactivity of the transition metals with dioxygen and "oxy-radicals" and therefore influence the apparent mechanisms by which oxidative damage to phospholipids, DNA, and other biomolecules is initiated. In biological systems the concentrations of redox-active transition metals capable of catalyzing these reactions appears to be relatively low. However, under certain conditions metal storage and transport proteins (ferritin, transferrin, ceruloplasmin, etc.) may furnish additional redox active metals.

Intravesicular pH and iron uptake by immature erythroid cells

The intravesicular pH of intact rabbit reticulocytes was measured by two methods; one based on the intracellular:extracellular distribution of DMO (5, 5, dimethyl + oxazolidin-2,4-dione), methylamine, and chloroquine and the other by quantitative fluorescence microscopy of cell-bound transferrin. The latter method was also applied to nucleated erythroid cells from the fetal rat liver. A pH value of approximately 5.4 was obtained with both methods and in both types of cells. Treatment of the cells with lysosomotrophic agents, metabolic inhibitors, and ionophores elevated the intravesicular pH and inhibited iron uptake from transferrin. When varying concentrations of NH4Cl were used, a close correlation was observed between the inhibition of iron uptake and elevation of the intravesicular pH. At pH 5.4 iron release from rabbit iron-bicarbonate transferrin in vitro was much more rapid than from iron-oxalate transferrin. The bicarbonate complex donates its iron to rabbit reticulocytes approximately twice as quickly as the oxalate complex. It is concluded that the acidic conditions within the vesicles provide the mechanism for iron release from the transferrin molecule after its endocytosis and that the low vesicular pH is dependent on cellular metabolism.

Vanadyl (VO2+) and vanadate (VO-3) ions inhibit the brain microsomal Na,K-ATPase with similar affinities. Protection by transferrin and noradrenaline

The activity of Na,K-ATPase was measured in brain microsomes as the function of increasing concentrations of vanadyl (VOSO4, V4+) and the vanadate (NaVO3, V5+) ions. Both forms of vanadium inhibited the Na,K-ATPase activity with high affinity -Ki (vanadate) = 3 X 10(-7)M and Ki (vanadyl = 1 X 10(-6)M. The stability of V4+ in ATPase reaction media (Tris buffers) was measured by electron spin resonance spectroscopy. Without any reducing agent, V4+ was quickly oxidised by atmospheric oxygen. When a reducing agent such as dithiothreitol was added, the V4+ was stable for at least 30 min and the inhibition pattern of Na,K-ATPase by V4+ was not changed. The blocking effect of V4+ in the presence of dithiothreitol was counteracted by pre-incubation with equimolar concentrations of transferrin or 100 times excess of noradrenaline. The regulation of brain Na,K-ATPase by vanadate may be represented by competition between low-capacity inhibitory binding sites localized on the enzyme molecule and high-capacity sites of intracellular proteins. Preferential binding of vanadyl to the latter type of sites will decrease the intracellular concentration of the free metal and thus eliminate the enzyme inhibition.

[Effect of pH and the binding anion on In3+-transferrin interaction: spectroscopic study of the perturbed angular correlation of 172-245 keV gamma rays of indium 111]

The formation of ternary complexes, transferrin-anion-In111 has been investigated by means of gamma-gamma coincidence spectrometry of the 172-245 keV rays. The angular correlation between the two gamma-rays emitted in cascade depends on the magnetic and electric fields gradients, consequently the chemical structure of metal holder. Any modification of this structure causes the variation of angular correlation. The study of G22 (infinity) as function of pH (G22(infinity): integrated perturbed angular correlation coefficient) has been performed to turn out the hydrolysis of In111 in aqueous solution, metal complex formation in presence of chelating agents (citric acid and sodium bicarbonate) and the formation of protein-metal complexes. The presence of complexing agents limits the domain of In111 colloid existence and allows fast transfer of ionised indium on the transferrin. Two types of metal-protein interactions has been turn out. The first in the weakly acidic range of pH is characterized by an affinity constant near to this of citric acid. The second lying in neutral and basic range of pH, where the formation rate of transferrin-In111 complex is fast (t less than 500 s). In citrate medium, for pH 6-7,5 the rate of metal transfer on the protein, studied by means of G22 (infinity) = f(t), is function of pH. The binding anion appears as an indispensable element for the formation of protein-metal complexes. The In111 previously chelated by 8-Hydroxyquinoline is fixed by the protein if only exits a binding anion in the solution. This mays bring in the formation of an intermediate active state, indispensable step for the ternary complex formation transferrin-anion-In111.

Plasma aluminum is bound to transferrin

Aluminum ion is bound to at least one of the two specific iron binding sites of serum transferrin and also to serum albumin, as shown by in vivo competition studies with 67-Ga, gel filtration chromatography and ultraviolet difference spectroscopy. Binding of aluminum to transferrin requires CO2 and therefore involves a specific iron site. Samples of commercial transferrin contained large amounts of aluminum. Aluminum may cause anemia by entering pathways of iron distribution and metabolism.

Transferrin support of stimulated lymphocytes

Transferrin was tested for its ability to replace serum in supporting mitogen and allogeneic cell stimulated human lymphocyte proliferation. Although transferrin, at concentrations greater than 5 microgram/ml, was incapable of completely replacing the serum used to support phytohemagglutinin, Concanavalin A, and pokeweed mitogen, stimulated human lymphocytes, in the absence of serum it significantly augmented the proliferative responses observed for mitogen, yet not allogeneically-stimulated cells. Augmentation is not due to a nonspecific protein effect and appears to be independent of the metal content of transferrin. The mechanism of growth support appears to involve an effect of transferrin following the G1 phase in the initial cell cycle.